Good write up, how stable are they? They look very tippy, especially with the jackstands. A little momentum and a rock on the floor could be trouble. I would imagine they are fairly safe with just the wheels on them, but with jackstands you are raising the c of g quite high. You really need to move the casters iout from under the wheel, like the other design where the tire sits in a cradle, there is no way that is flipping over.
That said, as long as they are your tools, and you know how to manage them, safety is in your court. Maybe don't let others use them though.